This invention relates to nuclear reactors having control rods reciprocable from above into a reactor core, and in particular to control rods reciprocable in guide tubes having a liquid flowing upward within the tube.
In a typical nuclear reactor the core is composed of a plurality of elongated fuel assemblies each containing a plurality of elongated fuel elements. A liquid coolant is pumped upward through the core in order to extract the generated heat for the production of useful work. The heat output of the core is usually regulated by the movement of control rods containing neutron absorbing material such as B.sub.4 C. In reactors of the pressurized-water type, each fuel assembly typically includes a plurality of cylindrical guide tubes through which cylindrical control rods are reciprocated. Some of the coolant flow is usually diverted into the lower end of the guide tube in order to cool the control rod, which generates heat in the nuclear transformation associated with its neutron absorbing function.
During typical power operation, most of the regulating control rods are maintained in a unique withdrawn position in which the lower tip of the control rod is within the guide tube at the upper end of the assembly. For reasons that are not fully understood, significant wear has been found on the inner walls of the guide tubes at precisely the elevation corresponding to the tip of the control rod in the withdrawn position. Flow tests on a laboratory model of the guide tube and control rod indicate that flow-induced vibration of the rod results in an oscillatory contact of the rod tip against the guide tube wall. It is believed that the source of the driving force is located well above the control rod tip, in the vicinity of the guide tube exit.
Continuous wear of the guide tube wall may lead to perforation of the tube and significant weakening of the fuel assembly frame. Thus, there has arisen the need to either eliminate the source of the flow induced driving force, or significantly mitigate its affect. Guide tube wear must be greatly reduced not only in nuclear reactors to be built in the future, but also in existing reactors.